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Sun-grazers, Sun-strikers and CMEs

Submitted by Karl Battams on Wed, 08/21/2013 - 07:52

Lately I have been rather tied up with Comet ISON, but when I'm not doing that, about a third of my time is spent administering the NASA-funded Sungrazing Comets, or Sungrazer, project. For those that aren't familiar, the Sungrazer Project is a citizen science program that enables volunteer astronomy enthusiasts from around the world to look for, report, and hopefully discover previously unknown comets in the images recorded by the ESA/NASA SOHO and NASA STEREO satellites. Since its inception in the early 2000's, the project has resulted in over 2,500 comet discoveries and more than doubled the number of officially designated comets!

Most of these comets are tiny, faint and unimpressive to say the least, but from time to time we get the occasional object that's a tad bigger than average and catches everyone's eye. Just a couple of days ago, August 18-20, we had such an object.

The animation, opposite, is a series of images recorded by the LASCO coronagraph instrument on the ESA/NASA SOHO satellite.

Two things should catch your eye: first, the comet in the lower-right corner, streaking towards the Sun, and leaving a long thin trail of debris behind itself; and second, a big "halo" coronal mass ejection blasting off from the Sun around the same time.

When we see events like this, I inevitably (and understandably) receive questions about whether the solar eruption was caused by the incoming comet. We've dealt with this many times now, and the answer so far has always been "no". Indeed it was about two years ago that Phil Plait, aka The Bad Astronomer, and I both simultaneously wrote about a large sungrazer that coincided with a CME in October of 2011.

I'm not going to regurgitate our points, and will instead encourage you to read the articles and also watch Phil's video about it. We both make a whole series of arguments about why that comet did not cause that CME, and every one of those arguments is completely valid and applicable to the comet from a day or two ago. Regardless of what we say there will always be tinfoil hat wearers who refuse to listen, but I'm always reassured by the number of people that ask perfectly valid questions and then write well-informed blog posts and articles about these events.

But what if a comet DID hit the Sun?

The bottom-line of our comets don't cause CMEs argument is that the SOHO comets never reach the solar surface, they're inconsequentially tiny compared to the Sun, and we know of no mechanism by which they could cause a CME. But we're scientists, and we like to toss ideas around, so it's fun to ask: what would happen if a comet hit the Sun?

I'm not the expert on this but I know a man who is: Prof. John Brown (Glasgow Univ.) . I've seen him give a couple of talks on the subject, and he wrote a fascinating (though math-heavy) paper titled "Mass loss, destruction and detection of Sun-grazing and -impacting cometary nuclei" that touched on this topic. In the paper, he looks at different sizes of comets and considers how each would behave were it on a sun-striking orbit. (Note: we have zero historical or modern record of any comet, ever, on a sun-striking orbit; this is all just hypothetical.) One of the more fascinating conclusions that John and his co-authors came to is that "in the case of impacts by the most massive comets (1020g or so) the cometary ﬂare energy release (2×1035erg) is much larger than that of the largest solar ﬂares
ever observed. An impact of this magnitude would have very significant terrestrial effects."

Could a comet cause a dangerous Earth-directed solar flare? Theory says yes, but probability is firmly on our side!

That's a rather terrifying statement, so let me calm some nerves here. First, when John says "massive comets" he means objects like Hale-Bopp, which was an exceptionally large comet. Comets that big are really rare inside the solar system. Second, recall we have never seen a comet strike the Sun, and there's no historical record of it ever happening. Third, and finally, the "cometary flare" John talks about would need to be Earth-directed, which gives it optimistically only about 20% (my estimate) of the solar surface to strike in order to give that result. So I'd say that asteroid impacts on Earth are of far more concern than unprecedented solar flares caused by unprecedented comets on unprecedented orbits hitting a bullseye on the Sun.

In that paper, the authors also note that "for all plausible Mo, q and physical parameters, nuclei are destroyed above the photosphere", where Mo is the initial mass of the comet, q is the perihelion distance (or closest approach to the Sun) for the comet, and the physical parameters refer primarily to composition and density of the comet. That sentence is their disclaimer that the Earth-threatening scenario they present is theoretically possible but for most plausible situations, the comet will be destroyed before it reaches the Sun's surface. Science has lots of those gotchas -- time-travel, to name one off the top of my head -- where we have theory versus reality. Still, it's tremendous fun to speculate on these issues so long as we keep our feet grounded in reality.

On a final note, I want to tie all this in to Comet ISON, which is why we have the website in the first place. We know that ISON is not a massive comet; we think it's in the range of ~0.5km - 2km. It's also certainly not on a sun-striking orbit, though will fly through the outer solar atmosphere. But keep the following in mind: coronal mass ejections (which are different to flares, btw!) happen a few times per day, every day during periods of high solar activity. The chances are thus pretty high that we'll see at least one CME in the hours surrounding ISON's perihelion passage in November of this year. This does not mean that it will have caused a CME! I'm sure I will need to quote this come November, so remember that you read it here first!

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